A problem in the manufacture of integrated circuits and planar circuit boards is the attachment of chips and other circuit components onto an electronic carrier, such as a substrate or board or printed circuit card, to name a few, and the making of interconnections between components attached to a common substrate.
A well known method for attaching components is to solder the component pins into holes drilled at regular intervals through the substrate (or card). This is commonly known as pin in hole technology (PIH). An example of PIH technology is shown in FIG. 1A, where a chip 30, having pins 31, is secured inside a hole 32, of a substrate 35, having electrical circuit lines 33. Problems with this technology are that it is not practical to attach components to both sides of the card and the components with associated pins take up a lot of space on the card.
An alternative method of attaching components and chips is by utilizing surface mount technology (SMT) in which components are attached to the surface of a printed circuit card. Holes are only drilled where necessary in order to make interconnections between wiring layers of the cards. An example of a component attached by SMT is shown in FIG. 1B, where the pins 31, of the chip 30, are secured to a pad 34, which is on the surface of a printed circuit card 35. Advantages of this system are reduced drilling costs and more card space for wiring. Components can be easily attached to both sides of the printed circuit card and can be mounted closer together. As the component lead-to-lead pitch is narrower in SMT than the pin-to-pin pitch in PIH, components can have more leads and therefore more functions. Furthermore many holes close together have a weakening effect on the mechanical strength of the substrate or board. Modern needs demand more dense circuit packages and with this comes more complex assembly processes.
Accordingly, another method developed for attaching chips 30, to a substrate 35, is direct chip attach (DCA). As with SMT holes are only drilled where necessary to make interconnections between wiring layers in the card. However, the chips have electrically conductive pads on their underside, rather than legs or pins, for connection. An example of a DCA chip is shown in FIG. 1C. The connections between the chip 30, and the substrate 35, cannot actually be seen as they are underneath the chip 30. These chips are typically attached to a circuit board by C-4 (Controlled Collapse Chip Connection) soldering, in which solder balls are formed or placed on the circuit connection pads. The leadless chip 30, is then placed on top of the substrate 35, and the solder is reflowed by methods well known in the art, such as, hot vapor reflow, infrared radiation, to name a few.
Another method of direct chip attachment is that of Direct Chip Attach Module (DCAM) in which the chips, such as, silicon chips, are mounted onto a substrate using soldering techniques, such as, C-4 soldering. The substrate in turn has solderable pads on its underside which are used to solder the DCAM to a circuit board, such as, by the use of a printed solder paste and some form of heat application.
DCAM enables increased circuit density and allows reductions in the size of printed circuit cards. However, one of the problems experienced with the assembly of DCAM components onto planar boards is that once attached, there are no easy visual ways of inspecting the quality of a solder joint beneath a component and the joints are very difficult to get to with a heat source to reflow the joint during repair. Also, if a component is bonded too close to the surface of the substrate, flux and other contaminants could be entrapped resulting in unwanted solder balls and voids and the necessary removal and reattachment of the component.
The method and structure of this invention produces an electronic circuit module which enables dense packaging of components onto a substrate with the added advantage that it provides for easy visual inspection of solder connections between the circuit module and an electronic circuit to which it is attached.